Modern terrestrial microwave radio systems provide a feasible technical solution for telecommunications transmission links at distances from some hundreds of meters to up to 80 km. Such systems are increasingly being developed in both cellular and fixed telecommunications networks. A radio link system is a particularly good solution in urban areas for wireless based networks in fixed telecommunications, and for base station interconnections and base station—base station controller in cellular communication. Unlike fiber, which can require several months for right-of-way and permits, microwave can be put into immediate operation. In addition, microwave easily goes over difficult terrain where cable cannot be laid, and microwave does not require trenching or pulling through duct work, which can take weeks or months, and which increases installation costs.
A typical microwave radio site consists of an indoor mounted base band unit, an indoor or outdoor mounted radio frequency transceiver, and an parabolic antenna.
Basically there are two types of radio link network topologies in use, namely star networks and ring networks. Of course, it is common for hybrid ring and star network as well as tree network to be deployed.
FIG. 1 depicts an example of a star network. It contains at least one telephone switch and one or more hub sites at strategic locations, which serve spurs or chains of subordinate sites from the centralized hub. The hub sites are connected to the switch via a transmission link, which usually is a trunk cable. A star network has one disadvantage in that outages on a single transmission link may affect many sites so lowering overall network reliability.
FIG. 2 shows a network configured in a ring structure. This structure requires some routing and grooming intelligence at all appropriate points in the network. The capacity of every link in the ring has to be sufficient to support all sites in the loop.
As mentioned above, radio link network provides one solution for realizing a cellular telecommunications network. Then, with reference to FIGS. 1 and 2, the switch might be a mobile switching center, the hub site can be a base station controller and a subordinate site is a base transceiver station. Each of the radio links performs a point-to-point connection.
FIG. 3 depicts a telecommunications system to which a radio link system using the invented method can be applied. The system is a cellular mobile network comprising a mobile switching center with a visitor location register, base station controllers 31 and 32, and several base transceiver stations BTS. The base station controllers 31 and 32 correspond to the hub sites as depicted in FIG. 2. Usually the base stations and their base station controller are interconnected with fixed trunk lines such as coaxial cables. For several reasons coaxial cable connections are not always possible. In this example, the base stations BTS1, . . . , BTS4 are each directly connected to the base station controller 31 with point-to-point radio links, so forming a star type topology whereas the base stations BTS6, . . . , BTS8 are connected to the base station site BTS5 with point-to-point radio links so forming another star. In this concept, the base station site means a single site which serves a plurality of hops from which one hop or link is common, like the radio link between BTS5 site and the base station controller 31. The base station controller 32 controls base stations BTS 9, . . . , BTS 118, which form a subsequent point-to-point chain. Mobile stations MS in a cell communicate with the network through the base station of that cell so that there is a radio connection between the MS and the BTS. The MS in cell 4 communicates with BTS 4.
One hop carrier between the base station and the base station controller can transfer four 2 Mbits channels each of them being divided into 16 kbits channels. Thus, one 2 Mbits channel can transfer 128 calls. Typical hop length is only 500 meters in the case the network is of the micro cell type.
A message, be it audio, video, or data is modulated on the microwave signal, which is often referred to as a carrier. The maximum distance between sites, also called a hop distance, is mainly determined by propagation characteristics of electromagnetic waves. The higher the carrier frequency the greater free space loss, or attenuation due to the atmosphere, i.e. the shorter the achievable distances. However, this also means that frequency re-use distances are shorter: the distance between links operating on the same frequency can be shorter without fear of interference.
There are three types of interference which should be considered in any terrestrial radio link network: 1) intrasystem interference occurs when a radio signal within a multi-hop network interferes with the receiver of a different hop, 2) external disturbance occurs when a foreign system affects a signal, 3) reflection—from anything that has a reflective surface can deflect other signals into the path of the transmitted signal and the stronger signal will interfere with the weaker signal.
Radio links have traditionally operated on regulated frequency bands which are further divided into frequency channels. The use of radio channels is regulated by local authorities and based on coordinated planning. Hence, in a predetermined local area in which radio links are to be established, only a predetermined overall bandwidth and then a predetermined number of channels are available for the radio links.
When a plurality of radio links or so-called hops are present within a given area, in the regulated radio environment, the channel choice is based on coordinated frequency planning. That is, the channel to be used for a specific radio link at a time is predetermined.
The objective of frequency planning is to assign frequencies to the radio links in order to avoid interference. Prior to the planning, it is essential to determine, at the earliest opportunity, what band are locally available for fixed link systems, and what the local “link policy” is. The majority of national frequency management administrations have some form of link policy regarding link lengths and net output power.
Recent developments in telecommunications have, however, lead to changes with regard to frequency allocations and have thus created possibilities to operate radio links and/or hops in non-coordinated frequency bands. These specific bands are left unregulated in the sense that selection of a working channel for an individual radio terminal inside the band is not controlled by the local authorities. Instead, the channel can be selected freely as long as the general requirements associated with the band are not violated.
As an example, European Telecommunication Standard ETS 300408 specifies the minimum performance parameters for radio equipment operating at frequencies around 58 GHz and not requiring coordinated frequency planning. Recently, the frequency band has been widened by ETSI to cover the band from 57 GHz to 58 GHz. Hence, it is possible to obtain 20 channels with the channel separation of 100 MHz. Within this band it is of interest to share the bandwidth among different links efficiently.
However, unlike the above described traditional radio links within a regulated (or coordinated) radio environment, those systems operating in an non-coordinated band will operate in an interference limited environment. That is, the signal quality of received signals may be deteriorated due to interference phenomena caused by neighboring radio links. Therefore, it is of increasing interest to consider how to share available bandwidths among various systems efficiently.
The most common way to avoid interference is to use different frequencies in the hops located near each other and reuse the frequencies at a distance. Hence, a great majority of the terrestrial radio link systems are based on the frequency division duplex (FDD) concept in which hops having a common site i.e. the hub site, are using different frequencies. Referring to FIG. 3, the channels between the base station controller 31 acting as a hub site and each of the base stations BTS 1, . . . , BTS4 could have a different frequency, for example. A duplex channel is formed from a frequency pair, one frequency of which is used for transmission and another one is used for reception. However, in FDD systems the same channel may be used by a hop pair in a hub site if the aerials are radiating to opposite directions.
A plurality of hops can use the same frequency if the system is based on a time division duplex mode of operation (TDD). In that case terminals in the hub site are transmitting only during predetermined transmit periods which are called time slots. The carrier frequency of each of the transmitters in the hub is the same but each of the transmitters has its own transmit time slot. Consequently, the interfering signal generated by the terminal varies greatly.
FIG. 4 illustrates a simplified block diagram of a hub site including several transceivers. In this example, the number of transceivers is the same as that of the base stations which communicate with the base station controller 31, see FIG. 3. In consequence, the hub site contains a transceiver A for communication with the BTS1 through an aerial 41, a transceiver B for communication with the BTS2 through an aerial 42, a transceiver C for communication with the BTS3 through an aerial 43, a transceiver D for communication with the BTS4 through an aerial 44 and a transceiver E for communication with the BTS5 through an aerial 45. Each transceiver is connected to its own aerial which in turn is aligned with the aerial at the opposite end of the link.
A straightforward implementation of the TDD principle in the radio link system would to allow each transceiver in a hub to use its own timing. This could be done if each hop uses a frequency which differs from frequencies of another hops. In that case, a transmission signal from a transmitter in the hub does not interfere with reception in the receiver of a neighboring transceiver due to the different frequencies.
However, a problem would arise if all the hops in a hub were using the same frequency. The reason for that is clearly comprehensible from FIG. 4. If the beams of aerials 41 and 42 were directed into substantially different directions, aerial 41 could send a signal at a frequency while aerial 42 were simultaneously receiving a signal at the same frequency. On account of divergent antenna beams radiation energy from aerial 41 would not significantly leak to aerial 43. Hence, interference at aerial 42 caused by aerial 41 would be negligible. However, in practice it is very likely that co-channel interference is too high so preventing usage of the same frequency.
The situation is remarkable worse if the aerials are facing in the same direction. Then radiating power from the sending aerial could leak to the receiving aerial causing high interference there. Therefore, due to its very high signal strength, the received interference signal could either saturate the receiver or even damage it. In case each terminal in a hub site uses its own burst rate and timing in transmission, the probability that one terminal is transmitting at the same time another terminal is receiving is very high.
The problem mentioned above leads to the fact that the interfering signal generated by a terminal is greatly varying with time. The hop density of radio links operating in a time division duplex mode in a frequency band, especially in the non-coordinated 58 GHz band, is limited not only by interference caused by the remote links, but also by considerable interference caused by the terminals located in the same hub site.